As used herein, a "semiconductor device" is a silicon chip (die) containing circuit elements on a face thereof, and a "semiconductor device assembly" is a silicon chip contained within a package and connected (wired or bonded) to conductive leads which exit the package.
A common type of semiconductor device assembly has a plurality of pins exiting a surface of the package body, and is termed a Pin Grid Array (PGA). FIG. 1 shows a typical PGA 100, which includes a square, flat ceramic package body 102 having a top surface 102a and a bottom surface 102b and an opening 104 which extends into the top surface of the package. The body 102 has a length L1 and a width W1. A plurality of pins 105 extends outward from the bottom surface 102b, and are electrically connected (not shown) to lead fingers (die bond sites) 106 which extend within the opening. Typically, the pins 105 are laid out in a square array pattern of rows and columns. The outer rows (and columns) of pins span a distance L2 (L2&lt;L1) lengthwise along the surface 102b of the package body 102, and span a distance W2 (W2&lt;W1) widthwise along the surface 102b of the package body. This type of PGA is commonly referred to as a "pins down" PGA.
It is a common practice to attach external chip capacitors 107 to the package to enhance its performance. FIG. 1 shows the typical locations for mounting (attaching) the capacitors to the top surface 102a of a pins-down PGA, namely, generally around the periphery of the top surface 102a of the body. Chip capacitors are typically soldered on to gold plated pads, on the top surface 102a of the PGA, which are electrically connected to some of the pins 105. Evidently, the chip capacitors 107 extend above the top surface 102a of the body.
The complexity of modern semiconductor devices results in a high number ("count") of pins. Pin counts in excess of one hundred are not uncommon. For high pin count packages, the pins are necessarily very thin (small diameter) and are closely spaced. Spacing on the order of 0.070-0.100 inch (center-to-center) is not uncommon.
FIG. 1a shows a pin 105, having a length H1, attached to the ceramic body 102 having a thickness of T1. Typically, the pin 105 has a core 110 made of Kovar (an alloy of cobalt, nickel, and iron) or Alloy 42 (an alloy of nickel and iron), both of which are ferrous materials. In manufacturing a ceramic PGA, pins are attached to the ceramic body by a copper-silver composite brazing material 111. Typically, the braze area 111 has a nominal diameter of approximately 0.028 inches. The pin, including the braze material, is then plated with approximately 60 micro-inches of nickel 112, then 60 micro-inches of gold 113. The nickel plating prevents the braze material from diffusing into the gold, while the gold plating enhances electrical conductivity and prevents corrosion. The physical integrity of the pins is critical for proper alignment with and insertion into corresponding holes of a printed circuit board or a socket to which the packaged device is ultimately mounted. Removal of the plating is also a common damage mode and would allow for unwanted corrosion of the pins.
FIG. 1b shows the layout of pins 105 as they extend from the bottom surface 102b of the PGA. A central area 124 of the bottom surface 102b is left void of pins to allow automated machinery to lift the package during fabrication of the semiconductor device assembly. This is a "partially" populated PGA.
FIG. 2 shows a PGA-type semiconductor device assembly 200 similar in some respects to that of FIG. 1. The PGA 200 includes a square, flat ceramic package body 202, of length L1 and width W1, having a top surface 202a and a bottom surface 202b. However, in this version, an opening 204 extends into the bottom surface 202b of the package. A plurality of pins 205 extends outward from the bottom surface 202b, around the opening 204 and are electrically connected (not shown) to lead fingers (die bond sites) 206 which extend within the opening. This type of semiconductor device assembly is commonly referred to as a "pins up" PGA.
FIG. 2a shows the top surface 202a of the pins-up PGA 200 and the typical locations for attaching chip capacitors 209. Both versions (FIGS. 1 and 2) of the PGA are applicable to the present invention.
As was the case with the PGA 100, in the PGA 200 the pins 205 are arranged in an array of rows and columns. Outermost pins extend a width W2 (W2&lt;W1) and a length L2 (L2&lt;L1).
FIG. 3 shows a typical semiconductor device assembly 300, very similar to the PGA 100. A semiconductor die 306 is inserted into the opening 304, and is mounted within the cavity with an adhesive (e.g. epoxy; not shown). The die 306 is then connected to the exposed ends (bond sites) of the lead fingers 303 by any suitable technique (e.g. wire bonding or tape automated bonding). The top surface 302a of the package has a metallic ring 305 formed about the periphery of the opening 304. After the semiconductor device 306 is mounted in the opening and connected to the lead fingers 303, a lid 308 is secured over the opening 304, "sealing" the package. The lid is essentially a flat metal (or ceramic) plate, and is evidently slightly larger than the opening 304. The lid is commonly sized to fit over the ring 305. A solder "perform" foil 307 of similar size and shape as the ring 305 is provided between the lid 308 and the ring 305, so that the lid may be secured to the package body 302 simply by heating the entire assembly, causing the preform 307 to seal and secure the lid 308 to the top surface of the package 302 over the opening 304.
At the completion of the fabrication (packaging) process, each semiconductor device assembly is individually inspected, under magnification, for various defects (e.g. scarred or gouged pins, contamination, etc.). Any devices exhibiting such defects are rejected and thrown away, and represent an undesirable expense. Therefore prevention of such defects is a noble objective.
During the process of mounting the die in the package, and sealing the package, the entire assembly is typically passed through automated processing equipment (e.g. to wire bond the semiconductor die to the lead fingers 303, inter alia), and through conveyor belt furnaces (e.g. to melt the preform 307, inter alia). The packages are commonly placed on boat transports ("boats") during these various packaging processes.
FIG. 4 shows a portion of a boat 400 of the prior art, of a type suitable for receiving a pins-down type PGA (e.g. 100). The boat 400 is typically an elongated metal structure capable of supporting and transporting a number (e.g. six) of packages 100. The boat 400 is a rigid metal structure having a platform portion 403, a top surface 404a, a bottom surface 404b, and an area within the dashed line 405 for supporting the semiconductor device assembly (100, 300). The boat is provided with two side leg portions 406, so that the top surface 404a is maintained a suitable distance above a transporting surface (e.g. conveyor belt; not shown).
A number of pin-receiving holes 408 are provided through the platform portion 403. The holes 408 are sized and spaced to allow the pins 105 to pass easily through the top surface 404a so that the package body (e.g., 102) rests directly on the platform 403.
Typically, the holes 408 are quite (about 0.020 inches) larger than the diameter of the pins 105, to allow easy insertion of the pins and to prevent gouging or scraping of the pin plating. However, this tolerance evidently allows the package 102 to move around, and allows the pins to contact the platform 403. Hence, some of the holes 408 are typically sized to be only slightly (e.g. 0.005 inches) larger that the pins 105, and these particular holes 408a act as guide holes, to accurately position the package on the platform and keep the package from shifting its position during packing operations (processes).
Evidently, the holes 408 and 408a are capable of gouging the typically softer plating material on the pins 105. Gouging and scarring of the pins arise during transport of the boat (i.e. from one process to another or within one process) when frequent vibration and jarring of the boat occurs and during the lifting and returning of the package during fabrication, as mentioned above. This problem is of great concern with respect to the braze area 120 (see FIG. 1a) where the effective pin diameter is at its greatest. This problem is also exacerbated by the smaller guide holes 408a on the boat.
As shown in FIG. 4, a larger cutout 407 extends through the platform portion 403 and is centrally located within the area 405. This cutout 407 allows automated process equipment to lift the semiconductor device assembly (100, 300) up off of the boat 400 during, and return it at the completion of, fabrication (packaging). The package is generally lifted once per assembly process (e.g. die attach, wire bond, etc.). Returning the package to the boat generates a violent collision between the package and the boat.
Additional cutouts 409 may be provided within the periphery of the area 405, to minimize the number of holes 408. Only relatively few holes 408, compared with the total number of pins 105 are needed.
Taking into account the cutout 407 and additional cutouts 409 the plate portion 403 is left with "bridges" 410 at two opposite sides of the area 405, and the bridges have a sufficient number of holes 408 to reasonably well align and maintain motionless a semiconductor device assembly 100.
As mentioned above, during the process of lifting the semiconductor device assembly and inserting the pins of the assembly into the holes 408, contact between the pins and the boat occurs because the boat is the direct support for the semiconductor device assembly. At the points of contact, motion of the semiconductor device assembly on the boat has been observed to cause the pin plating to be worn or scarred away, exposing either the under coat or the base metal, which can lead to undesirable corrosion.
FIG. 4a shows a portion of another embodiment of a prior art boat 420, of a type suitable for receiving a pins-up PGA (e.g., 200 of FIG. 2). Similar to the embodiment for the pins-down boat 400, this boat 420 is also an elongated metal structure capable of supporting and transporting a number of semiconductor device assemblies. The boat 420 is a rigid metal structure having a platform portion 423, a top surface 424a, a bottom surface 424b and an area within the dashed line 425 for supporting the semiconductor device assembly. The boat 420 is provided with two side leg portions 426 so that the top surface 424a is maintained a suitable distance above the transporting surface (e.g., of a conveyor or belt).
As shown in FIG. 4a, a large cutout 427 extends through the platform portion 423 and is centrally located within the area 425. A pair of corner "stops" 428 extend upward from the platform 423, outside the dashed line, at each corner of the area 425. Each stop 428 is punched and formed from the platform 423 (see exploded view in FIG. 4a) The stops 428 extend upward, perpendicular to the platform 423. The pins-up PGA rests on the platform area inside the corner stops 428 and the package body is positioned on the boat by the corner stops 428. Evidently, the pins of a pins-up PGA resting on the boat will extend away from (rather than through) the platform of the boat.
Hence it can be seen that in the prior art, two distinct boat transports (400,420) are required to handle pins-down and pins-up type packages (100,200). This involves designing and tooling one type of boat 400 for pins-up packages and another type of boat 420 for pins-down packages. This also involves keeping twice as many boats in inventory.
In any case, one of the main objectives in boats is not to damage the physical integrity of the pins. Prior art boats 400 for pins-down packages received package pins through an array of holes. Each hole is punched from the boat platform and left with a sharp edge. The array of holes minimize lateral movement of the package, relative to the boat. However, during the fabrication of a semiconductor device assembly, the package is lifted from and inserted into the boat frequently. During these operations the pins are commonly scratched, bent or scarred. Furthermore, collisions between the boat and package frequently occur during transport of the boat from one fabrication process to another. Contact points between the boat and package, in particular, the pin braze area, have often been observed to be scarred and gouged, removing the gold plating and exposing the underplating or base metal.
What is needed is a boat assembly that does not damage or package bodies. More specifically, what is needed is a boat assembly capable of supporting both pins-up and pins down PGA's and which does not contact the pins.